1. Field of the Invention
The present invention relates to an optical recording medium for recording and reproducing information using laser light and a manufacturing method thereof, and more particularly, relates to a multilayer recording medium having a plurality of information recording layers and a manufacturing method thereof.
2. Description of the Related Art
Heretofore, when a resin layer such as a cover layer is formed for protecting a recording layer in manufacturing of an optical recording medium, a spin coating method has been frequently employed since it is performed at a reasonable cost and is easily controlled.
In a Blu-ray Disc having a multilayer structure, a spin coating method has also been used; however, there have been problems in terms of film thickness accuracy.
Among the problems in film thickness accuracy, a phenomenon (hereinafter referred to as a “ski jump”) in which a resin protrudes along an outermost peripheral portion of a substrate has been very serious. FIG. 10 is a schematic view showing the peripheral portion of an optical recording medium formed by a spin coating. A height Δ101 of the protrusion of a resin formed along the peripheral portion is called the ski jump. When a cover layer is formed by a spin coating method, the film thickness accuracy is disadvantageously degraded particularly along the peripheral portion. In the cover layer, in addition to the variation in optical aberration caused by inferior film thickness accuracy, a problem in that the protrusion portion mechanically interferes with a recording/reproducing drive of the optical recording medium may arise.
In addition, recently, a medium having a plurality of recoding layers with at least one transparent resin-made interlayer interposed therebetween has been introduced on the market.
However, this interlayer is required to have a uniform thickness. In addition, in this interlayer, the generation of the ski jump also causes a problem similarly to the case of the above cover layer. In the case in which the film thickness accuracy of the interlayer is degraded because of the generation of the ski jump, a problem in that the information recording layers are difficult to be separated may arise.
In order to avoid this ski jump, various methods have been proposed.
For example, according to Japanese Patent Application Laid-Open No. 2004-288270, a method has been proposed in which the substrate is fitted in a recess formed in a tool which is in conformity with the substrate size so as to planarize the peripheral portion of the substrate. Since curing is performed in the state in which the substrate is fitted in the recess of the tool, the formation of the ski jump can be avoided; however, resin entering the space between the substrate and the tool forms a resin flash. Furthermore, when the substrate is recovered from the tool, new problems, such as chipping of the resin, may occur in some cases.
In addition, in order to avoid the problems described above, when the substrate is recovered from the tool in the state in which the resin is uncured, the ski jump may still occur due to the surface tension, and as a result, the original purpose to avoid the ski jump cannot be achieved.
Besides this proposal, a method has been proposed in which the ski jump itself is blown away by applying a nitrogen gas to the peripheral portion as disclosed in Japanese Patent Application Laid-Open No. 05-266472; however, for example, since UV irradiation is simultaneously performed, a general spin coating method cannot be used, and hence an exclusive complicated apparatus may be disadvantageously required.
As described above, in the related methods for avoiding the ski jump, new problems may arise or a complicated apparatus may be required.
That is, a method for solving the fundamental problems relating to the wettability of a surface onto which a resin is applied and the surface tension thereof have not been proposed at all.
Hereinafter, the reasons the ski jump is generated will be described.
When the surface has poor wettability, the resin applied thereto tends to form a spherical shape because of the surface tension thereof. Since the whole resin applied to a flat portion of a substrate tends to form a spherical shape on average, it forms a flat surface along the substrate as a whole; however, since the resin is not present outside the end portion of the substrate, the surface tension of the resin at the outermost periphery of the substrate becomes relatively large as compared to that of the other portions. Consequently, the resin in the vicinity of the outermost periphery protrudes, and as a result, the ski jump is formed.
As described above, heretofore, when the resin layer is formed by a spin coating method, there has been no method that overcomes the generation of the ski jump based on the consideration of the wettability of a surface onto which a resin is applied and the surface tension thereof.